Simpler fixtures

Now the fixture function automatically access the url resolver args and
kwargs without having to explicitly pass them in the condition:

Before:
        install_request_fixture('video', Video.objects.get)

        def condition(request, *args, **kwargs):
            ...
            some_object = request.fixtures.some_object(*args, **kwargs)
            ...

After:
        install_request_fixture('video', Video.objects.get)

        def condition(request, *args, **kwargs):
            ...
            some_object = request.fixtures.some_object
            ...

and `Video.objects.get` will be automatically called with `*args` and
`**kwargs`.